Senile macular degeneration is the leading cause of severe visual loss in the United States in people beyond age 60. Ocular histoplasmosis may be the second leading cause of visual loss in young and middle-aged living in the endemic area. In both disorders, visual loss most often results from the complications of choroidal neovascularization in the macula. The Macular Photocoagulation Study (MPS) is a multicenter, randomized clinical trial designed to answer the question: Is the use of argon laser photocoagulation to obliterate extrafoveal choroidal neovascularization of value in preventing visual acuity loss? The Macular Photocoagulation Study consists of two substudies: the Senile Macular Degeneration Study and the Ocular Histoplasmosis Study. Patients who meet the eligibility criteria and who agree to participate will have one eye randomized to argon laser photocoagulation or to "no treatment." Patients will be followed at six-month intervals for up to five years. Visual acuity will be used to assess the value of treatment versus no treatment. All data will be collected and analyzed at a central Coordinating Center, and all fundus photographs and fluorescein angiograms will be interpreted at the Fundus Photograph Reading Center. A Data Monitoring Committee will evaluate data several times per year and report its findings to the Executive Committee. Any significant findings will be reported promptly to participating patients, to referring ophthalmologists, and to the ophthalmic community. Clinical and photographic data of the fellow eye of each patient will be used to evaluate risk factors associated with the development of choroidal neovascularization and to further our knowledge of the natural history of these disorders.